multithreading in iOS

dispatch.m

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 1 * NSEC_PER_SEC), dispatch_get_main_queue(), ^{
    // Delay execution of block for 1 second.
});

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    // Execution of block in the background thread.    
});

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    // Execution of block in the background thread.

    dispatch_async(dispatch_get_main_queue(), ^{
        // Execution of block in the main thread.
    });
});

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    // Execution of block in the background thread.

    dispatch_sync(dispatch_get_main_queue(), ^{
        // Execution of block in the main thread.
        // Wait until the end.
    });
    
    // Continue execution in the background thread.
});

static dispatch_once_t task;
dispatch_once(&task, ^{
    // Single execution of block.
});

int times = 7;
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_apply(times, queue, ^(size_t index) {
   // Execution of block for 'times' times.
   // Parallelization with 'index' parameter.
   // Wait until all done.
});